Recently, with an increase in interest in environmental contamination and reinforcement of the regulation thereof, water treatment employing a membrane method using a filtration membrane excellent in completeness of separation or compactness has attracted attention. For use in water treatment, a filtration membrane requires excellent separation characteristic or water permeability and high mechanical strength.
In the past, filtration membranes formed of polysulfone, polyacrylonitrile, cellulose acetate, polyvinylidenefluoride, and the like using a wet or dry-wet spinning method were known as the filtration membrane excellent in water permeability. Such filtration membranes have a high-porosity and asymmetric structure through microphase-separating a polymer solution and coagulating the polymer solution in a non-solvent.
Among the materials of the filtration membranes, since the polyvinylidenefluoride resin is excellent in chemical resistance and heat resistance, it is suitably used as the material of a separation membrane. However, the filtration membranes formed of a polyvinylidenefluoride hollow fiber membrane having been proposed so far have a problem in that one of separation characteristic, water permeability, and mechanical strength is not satisfactory and a process of producing the filtration membrane satisfying all the characteristics is complicated.
A porous membrane in which a hollow braid is completely embedded in a semi-permeable porous membrane to enhance the mechanical strength has been proposed (Patent Document 1). However, such a porous membrane has a problem in that the water permeability thereof is low because it has a structure in which the braid is completely buried in the semi-permeable porous membrane and the porous membrane permeates most monofilaments.
On the contrary, a separation membrane has been proposed in which a hollow knitted braid is used as a support and a porous membrane is formed on the surface thereof to enhance both the mechanical strength and the water permeability (Patent Document 2). However, such a hollow porous membrane has a problem in that the porous membrane can be easily peeled from the braid because the porous membrane is disposed on only the surface of the braid. Since large macro voids exist in a membrane structure, there is a problem in that the separation characteristic easily deteriorates due to damage of external causes on the outer surface of the membrane.
The hollow braid used as the support is generally produced by a braiding machine. The braiding machine produces a braid by drawing out yarns from plural bobbins disposed upright on a flat panel, intersecting and knitting the yarns, and changing the positional relations of the yarns in a predetermined pattern by moving the bobbins along a predetermined path. The braid produced by the braiding machine and the hollow porous membrane using the braid as a support has the following problems.
Problem 1
Since the bobbins segmenting the yarns into small groups move in a complex way, the braiding speed of the braiding machine is low. Accordingly, there is problem in that the productivity of the support is low. When the productivity is low, the cost of the support increases, thereby causing an increase in cost of the hollow porous membrane using the support.
Problem 2
The braiding speed of the braiding machine is lower by one or more digits than the production speed of the hollow porous membrane. Accordingly, many braiding machines are required for supplying supports necessary for continuously producing the hollow porous membrane. When the yarn on a bobbin in the braiding machine runs out, it is necessary to stop the braiding machine and to carry out a yarn piecing operation such as replacement of the bobbin, incorporation of a new yarn into the braid, and cutting out ends of yarns protruding from the surface of the braid by the number of times of the number of bobbins (the number of strokes of yarns)×the number of braiding machines. Due to this complex operation, the cost of support increases, thereby causing an increase in the cost of the hollow porous membrane using the support.
Problem 3
To obtain satisfactory adhesive property of the porous membrane layer to the support, it is necessary to cause a part of the porous membrane layer to satisfactorily permeate the inside of the support. However, when the meshes of the braids are dense or the spaces between the monofilaments constituting the yarns are dense, a membrane-forming dope cannot satisfactorily permeate the meshes or fibers of the support when forming the porous membrane layer and thus the porous membrane layer is easily peeled out of the support.
On the other hand, a composite hollow fiber membrane without a defective portion of 10 μm or more has been proposed (Patent Document 3), which is obtained by applying a spinning dope including hydrophilic polymer as a non-solvent to a reinforcing material of a tubular knitted fabric in order to improve the separation characteristic. However, since the hollow fiber membrane has a dense layer, which affects the separation characteristic, only in the vicinity of the outer surface thereof, there is a problem in that the separation characteristic due to damage of external causes on the outer surface easily deteriorates. Since the non-solvent is added to the spinning dope, the spinning dope easily gels and the stability of the production process deteriorates. Since the hollow fiber membrane employs the tubular knitted fabric as the reinforcing material to enhance the mechanical strength but the permeation distance of the polymer resin membrane into the reinforcing material is set to be less than 30% of the thickness of the reinforcing material to maintain the water permeability while enhancing the mechanical strength, there is a problem in that the strength and the water permeability are not satisfactorily consistent in view of the peeling resistance of the polymer resin membrane from the reinforcing material.
On the contrary, a composite porous membrane has been proposed (Patent Document 4 and 5), which is difficult to peel out of a support and which is resistant to the damage on the outer surface thereof and has excellent stability in separation characteristic by providing two dense layers. However, since the porous membrane employs two membrane-forming processes to form two dense layers, the two layers may not be completely formed in a body. In this case, a gap may be generated between the layers and thus the outer layer may be easily peeled or damaged or defective portions of the layers may communicate with each other through the gap, thereby causing the deterioration in separation characteristics. There is a problem in that the two layers may not be completely formed in a body to maintain the water permeability and the water permeability and the peeling resistance are not consistent. A method of causing a membrane-forming solution with a low concentration of polymer to permeate important portions of a braid is employed to improve the peeling resistance between the support and the porous membrane layer. However, in this method, the adhesive portion, serving as a peeling-resistance agent, between the porous membrane layer and the support is only a fine porous portion permeating the fiber of the braid, this porous portion is formed of polymer with a low concentration to maintain the water permeability, and the peeling resistance can be improved, but there is still a problem with the strength. Since processes of producing and applying a diluted membrane-forming dope are required and two times of membrane-forming processes are employed, the production time is long and there is also a problem with production costs.